1. Field of the Invention
This invention relates to a resin composition curable by actinic radiation which does not substantially contain an acid group and has good adhesion, flexibility and other properties to be described hereinbelow.
2. Discussion of the Prior Art
Various photocurable resin compositions using ultraviolet light as the active energy radiation have been known heretofore. A typical composition comprises a polymerizable oligomer having at least two ethylenically unsaturated groups such as an unsaturated polyester or acrylated epoxy resin, a low molecular weight photopolymerizable difunctional, trifunctional, etc., cross-linking agent containing at least two ethylenically unsaturated groups (e.g., ethylene glycol diacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate (the number of ethylenically unsaturated groups, n=3 to 12), 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate and methacrylate homologues thereof, and divinylbenzene, divinyl sulfone, diallyl phthalate, diallyl benzene, diallylamine, and the like), and a photopolymerizable monofunctional compound, called a diluent (e.g., methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, benzyl acrylate and methacrylate homologues thereof, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, .alpha.-methylstyrene, vinyltoluene, vinylxylene, vinyl acetate, vinylacetic acid, vinylpyridine, and tetrahydrofurfuryl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, methoxy diethylene glycol acrylate, diethylaminoethyl acrylate, and their corresponding methacrylate derivatives), typified by the addition of 2-hydroxyethyl methacrylate or styrene to adjust the viscosity of the composition and change the rate of photocuring.
It is the common practice with such photocurable resins to increase the number of the unsaturated groups in the polymerizable oligomer or to increase the proportion of the polymerizable cross-linking agent and thus increase the number of cross-linkages in order to increase the rate of photocuring. However, as the cross-linking density increases, internal stress occurs in the photocured film and its adhesion to the adherend or substrate is reduced. Furthermore, the resulting coated film is very brittle.
It has been suggested to plasticize the coating and simultaneously increase its adhesion by incorporating a plasticizer in the resin composition, but this far from achieves the intended object.
Japanese Patent Application (OPI) No. 21205/1974 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a mixture of epoxy acrylate and an ethylenically unsaturated monomer, which scarcely adheres to glass, and its adhesion to metal surfaces is also weak.
Furthermore, the method for increasing adhesion disclosed in Japanese Patent Application (OPI) No. 125182/1976 in which a ketone resin is used in conjunction with a product obtained by esterifying one or more of the three protons of phosphoric acid to introduce an unsaturated group, for example, a .beta.-methacryloxy ethyl ester of phosphoric acid, suffers from several drawbacks. Since the phosphate ester is strongly acidic, the ester bond eventually tends to undergo hydrolysis and when a resin composition is actually formulated, hydrolysis occurs and the resin is degenerated and discolored. When this resin is coated on the surface of metal, degeneration of the metal surface is also observed. This is especially pronounced in the case of copper.
Resin compositions curable by the irradiation of light waves or other actinic energy radiation, such as electron beams, X-rays, .beta.-rays and .gamma.-rays (hereinbelow referred to as curable resins or simply resin compositions) must be stable and have good storability. Also, they should not bring about chemical changes on the surfaces of the substrates which they are coated upon. Furthermore, the cured coated films would have superior adhesion, flexibility, corrosion resistance, thermal stability, etc. As can be seen from the examples described hereinabove, none of known compositions are considered to meet all of these requirements.
Curable resin compositions having satisfactory storage stability, cured film adhesion and flexibility, and other properties have been investigated, and these investigations have led to the invention described below in detail.